1. Field of the Invention
The invention relates to a controlled atmosphere system for a refrigerated container and specifically to a system for regulating the amount of nitrogen, oxygen, and carbon dioxide inside a refrigerated container resulting in an atmospheric composition that extends post harvest shelf life/or quality of fruits and vegetables.
2. Description of the Prior Art
Controlled atmosphere containers for the post-harvest transport and storage of agricultural commodities are well known in the art. Such systems are usually used in conjunction with a refrigeration unit designed to lower the temperature of the interior of the container to a desired optimum temperature for the load being shipped therein. It has been established that temperature is the most important factor in controlling the rate of deterioration of fruits and vegetables. A controlled atmosphere system is considered as a supplement to proper temperature and humidity control.
The purpose of a controlled atmosphere system is to control the amount of oxygen and carbon dioxide inside the refrigerated container to change the rate of ripening of the produce stored in the container. The system controls the amount of oxygen (O.sub.2) and carbon dioxide (CO.sub.2) by replacing them with nitrogen (N.sub.2).
In early examples of controlled atmosphere systems for refrigerated containers, the modified atmosphere within the container was established, after loading, and not further modified during the period of storage or transportation. Problems with such systems included leakage both into and out of the container which changed the atmosphere. Subsequent technologies have provided for monitoring of oxygen and carbon dioxide levels within the container and have provided nitrogen and carbon dioxide sources to allow modification of the atmosphere during storage and transportation. A drawback to such systems was that supplies of carbon dioxide and nitrogen gas were required to maintain the desired atmosphere during normal lengths of transportation. Such gas sources were bulky and heavy and were not economically feasible to provide the gases required in a typical commercial application.
Systems have been developed wherein extremely high purity nitrogen gas is generated using an air separator of a type which incorporates membranes to divide a stream of ambient air, under pressure, into its principal constituents of oxygen and nitrogen. Some of such systems utilize an electronic controller to electrically operate a valve which will selectively increase or decrease the amount of nitrogen delivered from the membrane separator to the refrigerated container.
In such systems it is desirable for the controlled atmosphere system to be more widely used by unskilled operators. In order to facilitate this goal it is desirable to have a microprocessor controlled system which provides enhanced system performance, ease of setup and use and self-diagnostics. To this end it is particularly desirable to have a controller which operates according to a specific control algorithm including algorithms which energize the compressor, the heater, the system control valves and other system components in order to maintain the gas concentration inside the container box.